Why do so many people say not to have a combi boiler in a big house?

[ericmark]

I have lived in a house that circulates the DHW and yes it works in that you have hot water within seconds of opening the tap, however the pipes need to be well lagged, or it will heat the home in summer, also you don't want the water circulating at night so the pump needs a timed pump and clearly if the pump is not running you don't get fast hot water. So it ended up not being switched on in the summer as it got the house too warm as the insulation was not good enough.

Using hot water direct from a boiler be it an independent boiler or combined with the central heating unless so small heat store is included it means a further delay getting the hot water, if a heat store is included using a shower the water goes from cold to hot back to cold and then hot again as it uses up the heat store, and without the heat store you have to turns taps full on to get the boiler to cut in.

There is also the pressure, for potable water you don't want it to share same pipes as stored water, so you need the special mixer taps that only mix water as it leaves the faucet, and it often means non potable water in the bathroom to get pressure the same, so need bottled water to clean your teeth.

There are pros and cons with both systems, and water tanks do take up a lot of space.

I got rid of the hot water tanks in my old house, and fitted a Main 7 direct heat boiler, which resulted in being able to use a thermostatic shower in the bathroom, lost a little space in the garage, but gained enough space upstairs to make airing cupboard into a bedroom, yes we needed to run more water to get the hot water, but since higher pressure it came through just as fast. But it also stopped all ideas of heating the water using solar power, or back boiler, or immersion heater.

In this house shower is direct electric as not enough pressure to use thermostatic mixer, OK I know there are tanks today which can give mains pressure to the hot water, but they are expensive, and if you already have a tank then to renew tank and boiler gets rather expensive.

Mother house had the two boilers combined, the combi boiler, mine was two independent boilers, but only difference was her boiler was well over sized for the house, not sure why it was so big, mine I seem to remember was 18 kW plus 18 kW for house, hers was 28 kW with single boiler, and both houses had the same problem, you needed large shower heads or in summer the boiler would cut out.

Before the combi was fitted to mothers house she had a power shower, only way to get enough pressure.

I now don't have the option, oil boiler don't modulate like gas, so have to use a water store, the work in fitting return pipes means the circulating pump is a non starter, and having 22 mm pipes for hot water means I run a lot more cold water than I would with a mains pressure hot water supply with 15 mm pipes.

This was part of the problem with last house, when the main 7 was fitted it no longer needed 22 mm pipes, but they were already fitted, so took ages to get hot water to kitchen sink. When mothers house was done, all the old pipes were ripped out, and new pipes fitted, so with the smaller pipes water reached hot taps much faster than before combi was fitted.

To start from scratch if you have the room, then a nice new storage tank with mains pressure hot water with 15 mm pipes and circulation pump would be better than a combi, but also a lot more expensive, if reusing what is already fitted then much depends what is already fitted. And as to circulation pump you really do need to insulate the pipes well or it heats house in summer, and if you do insulate well not as much need to circulate anyway.

I will admit I am considering a local hot water store for the kitchen, takes to long to get hot water through, if I had a local hot water unit in kitchen then in summer I would not run the boiler, shower is instant electric and to heat 40 gallons of water do I can wash my face with warm water in the morning I don't think so. I would just have a shower.

 

[ericmark]

duplicate

 

[oldbutnotdead]

Combi boilers are ideal for tiny modern flats or houses where there isn't room for any fripperies like an airing cupboard or header tanks. Condensing boilers (which all modern boilers are in this country) are more efficient than older non-condensing types when running within their design parameters (low return temperature). Modern boilers also increase efficiency in the same ways as modern cars (through lots of complex electronics which are fab when they work but expensive to repair if/when they fail).
The whole point of having more than one bathroom is so that more than one person can be showering/bathing at the same time (same as having multiple toilets). To me it seems foolish to have more than one bathroom and not be able to use both/all of them at the same time.

 

[ericmark]

I will ask a question, when a boiler senses that the house is warm it turns down the temperature of the circulating water, this is the whole idea of having a modulating thermostat controlling boiler, and having cooler water circulated means less losses and less hysteresis.

Now when a combi boiler gets a demand for hot water it turns the temperature up/down to suit DHW and temperature we want the water out of the taps, so the CH water can be a completely different temperature to the DHW.

So with a water store how does the system tell the boiler to change temperature when heating up the DHW to when working the CH? Clearly we can reduce the DHW temperature to CH temperature during heart of winter, but in autumn and spring we likely want DHW hotter than CH how is this done?

Even with a hot water tank, two of the three showers I have are direct electric, also I have two baths, however can't possibly fill two baths, and after filling one, the time to reheat water is over an hour, so unless first user is seriously prunes, they may as well use same bath. I am sure I could fill two baths quicker with a combi boiler than I could with a water storage tank.

Yes I know you can get special systems with extra large tanks, and super efficient heat exchangers and you could have two independent boilers one to heat DHW and other for CH, but we are not talking about a hotel, we are talking about a large house like mine with two bathrooms and a shower room I am assuming, and the only way I can use all three together is to have a instant electric shower in at least one. As said in my case no option as oil fired, but with gas it is completely different, you can turn down the output of a gas boiler from 28 kW to 6 kW you can't do that with oil.

 

[fezster]

So with a water store how does the system tell the boiler to change temperature when heating up the DHW to when working the CH?

You can't using standard controls. Hence the compromise.

With manufacturer specific controls (eg. Vaillant Ebus) or controls which use OpenTherm, this is possible.

 

[ericmark]

You can't using standard controls. Hence the compromise.

With manufacturer specific controls (eg. Vaillant Ebus) or controls which use OpenTherm, this is possible.

So to use a system boiler you in real terms must use one with ebus controls, so what is the cost implication? In my mothers old house I used a on/off thermostat in the hall and an electronic TRV head set so the thermostat would only turn off on warm days, basic idea was to stop the boiler cycling on warm days, the thermostat would not turn off in winter, and control of each room was with TRV's and main rooms had programmable heads, so the control cost £35 for hard wired programmable wall thermostat (Flomasta 22199SX) running same program as the TRV heads, and £80 for 8 x electronic TRV heads (Eqiva eQ-3) so whole system would cost at today's prices £115 with all rooms using programmable TRV heads. Not including cost of bases as all systems will need the bases, radiators, etc.

So not sure on cost of all OpenTherm thermostats, Nest (£200), Drayton Wiser (£140) and EvoHome (£250) the latter needs an add on to work with OpenTherm and needs at least some EvoHome heads. But at least it would need same TRV heads as the simple system so at least £80 for heads.

Also £500 plus for the water tank, yes simple one £200, but comments were if two bathrooms then need to be used together, so a fast heat exchanger is required, and the cheap ones don't have a fast heat exchanger. Plus circulating pump, and insulation around all the pipes, or there is no advantage, so I would on a rough estimate say to fit a system boiler which actually uses the advantages available to a system system it will cost an extra £1000.

No point using a system boiler if not getting the advantages offered by it. So I invite you to show I am wrong.

 

[johnny2007]

I have a bosch 28i and 3 bathrooms on 3 levels all with thermostatic bars.
The only slight problem is that to get hot water in the top floor you need to run the tap for almost a minute.
Not a problem for us.
Pressure up there is same as other levels and for some weird reasons we never had 2 showers running at the same time.

 

[bernardgreen]

to get hot water in the top floor you need to run the tap for almost a minute.
Not a problem for us.

Other than the expense of the gas that is wasted during that minute. Also the waste of water

 

[johnny2007]

Other than the expense of the gas that is wasted during that minute. Also the waste of water

Marginal in the great scheme.
I don't think I ever used hot water on the top tap and an extra minute of running shower will not break the bank (will only upset Greta and her fans, the Gretins)

 

[durhamplumber]

understand a combi would struggle to supply more than one bath / shower but how often do two people in a house have a bath or shower at the same time?

That is why.!...People do come on here making out life is not worth living unless two showers can run at once in their house.

 

[bernardgreen]

making out life is not worth living unless two showers can run at once in their house.

It is when one person is in the hot shower and another person starts to wash up the dishes that life may not be that good.

Not too bad if the shower goes from hot to warm, but if it goes from hot to cold then the effect on the body in the shower can be drastic.

 

[ReJect]

What a lot of people, (including many plumbers) don’t consider is you can use a combi gas boiler and have hot water storage cylinder (vented or unvented).
Just use combi hot supply for supplies fairly close by, like kitchen sink hot or nearby bathroom, etc. Then you pipe a typical S plan with pipes to cylinder.
Ideally a house should be designed with consideration of boiler position and bathrooms back to back and kitchen in same area, or directly above and below to avoid long distances runs of pipes.
Oil combis best avoided as they are expensive and troublesome